1. Field of the Invention
This invention relates to an improved syringe and, more particularly, to a dual-chambered syringe having a fluid reservoir accessible to a delivery chamber upon axial displacement of a plunger.
2. Background Discussion
In many medical procedures there is a need for locally anesthetizing a large region subject to an operation or other treatment. Local administration of anesthetic may require several distinct injections in the affected area, or injection of a first volume of anesthetic at a relatively shallow depth followed by one or more further injections after advancing the hypodermic needle deeper into the tissue of the patient, or both. This successive delivery of anesthetic to varying tissue depths ensures a complete and measured numbing of all of the nerve endings in the treated region.
When delivering a large quantity of any fluid using a syringe, such as in the successive, varying depth injection method, the syringe must have a large capacity, or must be withdrawn intermittently and refilled. Large syringes are cumbersome, and it is often difficult to meter small quantities with them as the barrel has a large cross-section. Thus, even a small movement of the plunger correlates to a large fluid displacement. Even smaller syringes having volumetric graduations used to inject two or three sequential doses are difficult to meter exactly. Moreover, the smaller the diameter of the syringe, the less pressure is required to deliver the injection, and consequently, less pain. Withdrawal of the syringe from an injection site for refilling and subsequent re-injection is inconvenient and increases the chance of infection and trauma to the patient.
There have been efforts in the prior art to provide a dual-chamber syringe for storing a volume of fluid in a reservoir to be transferred into a delivery chamber, or visa versa. Typically, the fluid is ejected from the delivery chamber through a nozzle by a plunger. There are various motivations for providing dual-chamber syringes, including simply increasing the syringe capacity, providing a sterile self-contained single-use syringe, and mixing two components prior to injection, for example. Many of the known devices, however, require the injection nozzle to be plugged prior to transferring fluid between the reservoir and the delivery chamber. In other known devices, one chamber is placed in communication with another only upon relative rotation of the chamber barrels.
One dual-chamber syringe is shown in U.S. Pat. No. 553,234 issued to Finot. This syringe includes a rotatable inner chamber for selectively communicating the inner chamber with either the injection nozzle or the reservoir. Despite the benefit of a larger carrying capacity, the syringe must be manipulated with two hands to transfer fluid between chambers. Further, the syringe of Finot was not designed for successive, varying depth injections which are best accomplished rapidly and with minimum lateral movement.
Despite numerous previous dual-chamber syringe designs, all share the drawback of requiring a two-handed operation to transfer fluid between the internal chambers. There is thus a need for a dual-chamber syringe particularly suited for successive, varying depth injections which can be actuated with one hand.